1. Field of the Invention
The present invention relates to a method of measuring non-linearity. More particularly, the invention relates to a method of measuring non-linearity in magnetic recording/reproduction by recording/reproducing digital data, as a change in the polarity of magnetization, into or from a recording medium such as magnetic tape, magnetic card, flexible disk, magnetic disk, magneto-optic disk or magnetic drum.
2. Description of the Related Art
Accompanying the trend toward higher densities and higher data transmission speeds of magnetic recording/reproducing devices in recent years, it is becoming essential to measure a non-linear transition shift (NLTS) that occurs in the magnetic head, recording medium and record/reproduction transmission system. The NLTS is a data necessary for magnetically recording data in the recording medium and for correctly reproducing the recorded data by taking the transition shift (TS) into consideration, so that the recorded data will not be affected by the preceding or succeeding recorded data.
A prior method of measuring NLTS has been disclosed in a theory entitled “A Generalized Frequency Domain Non-linearity Measurement Method” by X. Che, M. J. Peek and F. Fitzpartick, IEEE Transactions on Magnetics, Vol. 30, No. 6, p. 4236, 1994. This theory teaches a method of measuring NLTS that occurs when signals of a bit-string pattern, in which the magnetic inversion occurs continuously over two bits (dibit), are magnetically recorded in the medium.
The method of measuring the NLTS will now be described. The method of measuring the NLTS comprises the following three steps.
In a first step, the data are recorded into the medium using reference signals which repeats a bit-string pattern of 30 T with the pulse width of each bit being T and the polarity being inverted every after 15 bits (15 T), and a fifth harmonic component of the reproduced signals (hereinafter referred to as fifth component) is measured. The fifth component is denoted by V5ref. The reference signals are expressed below by the NRZ notation which expresses the codes for recording the data in the medium in terms of the levels and by the NRZI notation which expresses the codes for recording in terms of the inversion of levels.
Reference signals:NRZ notation:111111111111111000000000000000NRZI notation:100000000000000100000000000000
In a second step, the data are recorded into the medium by using the to-be-measured signals which repeat a bit-string pattern of 30 T including a Dibit pattern, and a fifth component V5pat of the reproduced signals is measured. The to-be-measured signals are expressed below by the NRZ notation and by the NRZI notation.
To-be-measured signals:NRZ notation:100000001111111011111110000000NRZI notation:110000001000000110000001000000
In a step 3, a ratio Vab=V5pat/V5ref is calculated from the fifth component V5ref and from the fifth component V5pat measured in the steps 1 and 2. Then, the NLTS (Dibit) occurring with the Dibit is calculated from the following formula (1). Here, NLTS is a value normalized with 1T, i.e., a value of when the pulse width of 1T is set to be 1 (100%).NLTS(Dibit)=a cos [(2−Vab2)/2]*3/π  (1)
The above formula (1) can be approximated by the following formula (2),NLTS(Dibit)=Vab*3/π  (2)
The above theory further describes a method of measuring the NLTS of when the inversion of magnetization further continues. In addition to the above theory written by J. Fitzpatrick, the theory by A. Taratorin, S. X. Wang, B. Wilson entitled “Non-Linear Interactions in a Series of Transitions”, IEEE Transactions on Magnetics, Vol. 33, No. 1, p. 956-961, January, 1996, describes that when a reproduction head of the MR (magnetic reluctance) type is used, measurement of NLTS involves a large error unless non-linearity of the reproduction head is removed.
As one of the NLTSs, further, there can be exemplified a transition shift by the recorded data (HTS: hard transition shift or O/W (overwrite) NLTS) and as the measuring method thereof, there has been known a method that calculates the NLTS from the reference signals (f) repeating a single bit-string pattern and from the to-be-measured signals (f/2) having a frequency one-half thereof. According to this method, it is necessary to overwrite a to-be-measured signal (f/2) on the medium synchronized with a reference signal which is previously recorded on the medium, which requires a long time. Therefore HTS (=NLTS) often differs when the frequency of the reference signals is changed and it is obliged to change the measuring method unlike the above-mentioned method of measuring the NLTS relying upon the fifth harmonic component. In other words, the measuring method loses compatibility.
In the past, it was sufficient to determine the performance of a device for magnetic recording/reproduction by only measuring the NLTS that occurs when a dibit bit-string signal is magnetically recorded in a medium. However, recently, accompanying the trend toward higher densities and higher data transmission speeds of magnetic recording/reproduction devices, it has been required to measure the NLTS that occurs when a tribit bit-string signal or a 2T bit-string signal other than the dibit bit-string pattern is magnetically recorded repeatedly in a medium.
So far, further, the reference signals and the signals to be measured have been given by a host equipment, and the NLTS has been calculated by the host equipment. Therefore, the host equipment side requires a special function, and it is difficult to calculate the NLTS again after shipment.
In view of the above-mentioned problems, therefore, it is an object of the present invention to easily measure the various kinds of NLTS without taking a long time while decreasing error in measuring the non-linearity caused not only by the electromagnetic induction-type reproducing head but also by the MR (magnetic reluctance)-type reproducing head or imparting compatibility to the method of measuring NLTS, i.e., without changing the measuring method in measuring the non-linearity in the magnetic recording/reproduction.
Another object of the present invention is to measure the NLTS by using a magnetic recording/reproduction device alone.